The present relates to imaging array integrated circuits. More particularly, the present invention relates to imaging arrays having focal plane phase detect pixels and methods for performing focal plane phase detection in such arrays.
Imaging arrays having focal plane phase detect (FPPD) pixels are known in the art. FPPD pixels collect light selectively from either left or right side of the camera lens. FPPD pixels are always placed in pairs adjacent to each other. The pairs are distributed over most of the pixel array, typically over about 80 percent of the area around the center of the array, leaving the edges clear. The density of the FPPD pixel pairs is a few percent (1-3) of the pixels within that central area.
The most common method used to implement FPPD pixels in imaging arrays is to employ metal layers disposed in layers above the sensors to shadow selected portions of the FPPD pixels. FIG. 1A shows exemplary prior-art FPPD pixel sensors employing metal layers disposed in layers above the sensors to shadow selected portions of the FPPD pixels. FIG. 1A is a cross-sectional view of a right FPPD pixel sensor 10a and a left FPPD pixel sensor 10b. Pixel sensors 10a and 10b are n-type diffused regions formed in a p-type semiconductor substrate 12 as is known in the art. An interlayer dielectric layer 14 is formed over the surface of the substrate 12 and the pixel sensors 10a and 10b. Microlenses 16a and 16b are formed, respectively, over pixel sensors 10a and 10b. A metal segment 18a is formed and defined over pixel sensor 10a and is positioned to block light from entering on the left side of the pixel sensor 10a. A metal segment 18b is formed and defined over pixel sensor 10b and is positioned to block light from entering on the right side of the pixel sensor 10b. 
Another technique that has been suggested for creating FPPD pixel sensors is to deposit an opaque silicide layer over selected portions of the FPPD pixels. FIG. 1B shows exemplary prior-art FPPD pixel sensors employing an opaque silicide layer over selected portions of the FPPD pixels to shadow selected portions of the FPPD pixels. Many of the features of the pixel sensors are the same in FIGS. 1A and 1B and will be identified using the same reference numerals.
FIG. 1B is a cross-sectional view of a right FPPD pixel sensor 10a and a left FPPD pixel sensor 10b. Pixel sensors 10a and 10b are n-type diffused regions formed in a p-type semiconductor substrate 12 as is known in the art. An interlayer dielectric layer 14 is formed over the surface of the substrate 12 and the pixel sensors 10a and 10b. Microlenses 16a and 16b are formed, respectively, over pixel sensors 10a and 10b. An opaque silicide layer 20a is formed and defined on the surface of the diffused region forming pixel sensor 10a and is positioned to block light from entering on the left side of the pixel sensor 10a. An opaque silicide layer 20b is formed and defined on the surface of the diffused region forming pixel sensor 10b and is positioned to block light from entering on the right side of the pixel sensor 10b. This technique has proved to be unsatisfactory since the silicide layers create significant amounts of leakage current in the FPPD pixels.
As digital cameras become thinner, the angles of light irradiating the individual pixel sensors in the imaging array become larger as measured normal to the surface. Designers have employed several techniques to accommodate these angles.
According to one possible solution, the pixel sensors that make up the array can be increased in size at the cost of decreasing resolution. This is generally not considered to be a satisfactory solution in view of the trend to increase rather than decrease the resolution of digital cameras.
In very small pixel sensors, such as those used for cell-phone camera sensors, a “light pipe” has been employed. This is similar in concept to a fiber optic cable, relying upon total internal reflection (TIR). It therefore requires the use of a high-index polymer as the core of the light pipe. The concept will work well for small incident angles (steep angle of incidence on the sidewall), but it becomes progressively less useful as incident angles increase. According to one particular prior-art light-pipe solution shown in FIG. 2, light pipes employing internal reflection at the edges of lenses are positioned over the pixel sensors. Adjacent pixel sensors 10a and 10b are shown formed in p-type substrate (or well) 12. Dielectric layer 14 is formed over the pixel sensors 10a and 10b. Lenses 16a and 16b are formed on the surface of the dielectric layer as is known in the art.
Unlike the pixel sensors depicted in FIGS. 1A and 1B, vias are formed in the dielectric layer, respectively over and in alignment with pixel sensors 10a and 10b and are both filled with a polymer to form light pipes (indicated at reference numerals 18a and 18b) having a high index of refraction (e.g., n≅1.6). A layer of material (shown by reference numerals 22) provides total internal reflection is formed at the edges of the lenses 16a and 16b between adjacent pixel areas.
Light rays directed at the surface of the pixel sensor array containing pixel sensors 10a and 10b, two of which are shown symbolically at reference numerals 24. As shown in FIG. 1, the light rays bend at the interface of the lenses 16a and 16b. The light rays 24 are also shown reflecting from the layer 22 at the edges of the lenses. Without the presence of the layers of material 22, these light rays 24 would continue along a path that would lead into the next adjacent pixel but the presence of the layer of reflective material 22 reflects them back into the pixel area into which they entered.
As the light rays 24 continue downward from the lens into the polymer layers 18a and 18b, they are reflected by the interface (shown at reference numerals 26a and 26b) between the respective polymer layers 18a and 18b and the dielectric layer 14 (having an index of refraction of about n=1.53) in which they are formed. This interface is not 100% reflective and so some of the light shown in dashed lines at reference numerals 28 passes through the interface, through the dielectric layer separating the two adjacent pixels, and undesirably into adjacent pixel sensors causing undesirable crosstalk.
Ideally, it would be desirable for a small pixel to have the same acceptance angles as a large pixel without the aforementioned drawbacks of the present solutions. It would also be desirable to provide a light pipe pixel sensor array that both accepts light from relatively large angles and includes FPPD pixels.